1. Technical Field
This invention relates to the field of illumination systems and more specifically to lighting equipment where it is advantageous to have a minimum amount of light outside a predefined cutoff angle.
2. Background Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
As is known, a luminaire is a device to produce and distribute light wherein the production of light can comprise artificial or natural light sources. An example of a reduced-glare luminaire is shown in Lewin, U.S. Pat. No. 3,988,609, wherein a plurality of fluorescent lamps are housed in a reflective cavity, and whose output distribution is limited by way of a prismatic film spaced at a distance from the lamps. The film has a smooth side facing the lamps and varying prismatic features on the opposing face. From the perspective of a prismatic feature, light rays can be received from any of the lamps as well as from reflection off the reflective cavity. Further, light rays that travel at near-grazing-incidence to the smooth face of the film are likely to undergo total internal reflection (TIR) once striking the face of a prism (see Whitehead, U.S. Pat. No. 4,262,220). The net effect is that a single prism will see a wide variation of incidence angles, and thereby unable to constrain the light rays within the glare free angular extents, and further would suffer from efficiency losses due to absorption from reflections off the cavity wall and rays that recycle back to the lamp. Thus Lewin, while reducing some glare, still allows glare and also considerably reduces the efficiency of the lamp.
With respect to objectionable glare, standards have been developed to assess a luminaire, such as the Deutsches Institute for Normung (DIN, specification entitled “Artificial Lighting, Terminology and General Requirements”, which is now International Standards Organization (ISO) Specification 5035, (herein after referred to as “the DIN Specification”). The DIN Specification requires, among other things, restricting light emanating from a luminaire between 45 and 85 degrees relative to a ceiling normal. It is known that limiting light rays in such a manner will reduce glare in the vicinity of computer screens thereby decreasing eyestrain and fatigue on office workers.